Hepatitis C virus (HCV) is an important human pathogen that can cause severe liver diseases including liver cirrhosis and liver cancer. The long- term goal of this project is to understand the molecular mechanisms that regulate the replication of HCV. Our previous research demonstrated that the HCV core protein could dimerize and interact with the E1 envelope protein, which had also been shown to interact with the E2 envelope protein. These interactions between HCV structural proteins are likely very important for the maturation of the HCV virion. Recently, a cell culture system that allows HCV RNA replication has been developed. However, this cell culture system does not allow the studies of viral morphogenesis due to the lack of HCV structural protein genes in this system. As part of our effort to understand HCV morphogenesis, we will perform cis and trans-complementation experiments to improve this cell culture system for the production of HCV virions. Our goal is to use this system to study HCV gene functions and morphogenesis HCV is a positive-stranded RNA virus with a genome size of 9.6 Kb. This genomic RNA produces a polyprotein, which is cleaved by viral and cellular proteases to generate to generate at least ten viral gene products. Our recent studies indicated that a 17 kDa (p17) was also produced from the HCV genome by ribosomal frameshift. Our goal during this grant period is to continue our previous studies to investigate the biological functions of this novel HCV gene product and to understand the mechanism that regulates its expression.